The invention relates to a hybrid circuit comprising an input, an output and an in/output, at least one signal transistor, at least one auxiliary transistor incorporated as a current multiplier in a circuit with an associated signal transistor and an impedance circuit incorporated in a circuit with associated signal transistor, a junction point of this impedance circuit being connected to the output of the hybrid circuit, the impedance circuit as a whole having such an impedance related to the coefficient of the auxiliary transistor that the hybrid circuit fulfills the line matching condition.
A hybrid circuit of the type mentioned hereinbefore is known from the German "Auslegeschrift" No. 2 829 392.
The active hybrid circuit known from the above-mentioned publication can be designed to be symmetrical, having two axially symmetrical half sections. This known active hybrid circuit can further be designed to comprise only NPN-transistors in the signal path, as a result of which in accordance with the then state of the art higher limit frequencies could be obtained than if PNP-transistors would have been inserted in the signal path.
If the known hybrid circuit is of a symmetrical configuration, each half section comprises two signal transistors, one connected to the input and in/output of the hybrid circuit and a negative supply line and being inserted in a circuit comprising an auxiliary transistor switched as a current multiplier, the auxiliary transistor coefficient being smaller than 1, the other signal transistor being connected between the output and in/output of the hybrid circuit and a positive supply line and being incorporated in a circuit comprising an impedance circuit, in the simplest case composed of a series arrangement of two resistors, with more specifically a first resistor being inserted between the collector of the aforesaid signal transistor, and the positive supply line and having a value amounting to half the line resistance, for example 300 Ohms, and the other resistor being connected between the collector and the base of this signal transistor and having a value equal to the inverse decreased by 1 of the aforesaid coefficient times half the line resistance, in order to meet the requirement of line matching. However, to avoid cross-talk of the input signal at the output of the hybrid circuit the aforesaid resistors of the impedance circuit are not interlinked directly, but by means of a base emitter transistor of an additional transistor of the impedance circuit, whose collector is connected to the positive supply line.
The object aimed at with the known hybrid circuit is transmitting onto the line the whole signal current at the input of the hybrid circuit, this object being achieved through the measures of current amplification (division) and impedance transformation, at least as regards transmitting onto the line. Further advantages of the known hybrid circuit are that it is readily integrated relatively simple to construct and that it has a small current consumption.
In addition to the advantage of high limit frequencies, the aforementioned advantages are of importance when such a hybrid circuit has to be inserted in an integrated modem circuit for example, for a viewdata system.
For such use, however, not only should the current consumption or the dissipation be as low as possible, but especially for direct supply from the line the required supply voltage should be as low as possible, with for example a total available power of approximately 20 mW and a total available supply voltage of approximately 4.5-8 V.
Unfortunately, with the known hybrid circuit, specially in practical embodiments thereof, this is not possible owing to the required transistors connected in series and in a totem pole configuration between the supply lines, each requiring a diode voltage (base/emitter voltage) of approximately 0.7 Volt.